The general problem of preventing the transmission of vibration and other movements from one body to another dates back to the dawn of the machine age. The development of increasingly complex machines has resulted in the ubiquitous utilization in such machines of any of various approaches to solving this problem. Increases in the accuracy of tasks performed by various machines have demanded increasingly sophisticated or more tailored approaches to reducing transmission of vibrations and the like from one portion of the machine to another and/or to a workpiece upon which a machine is performing a task. Also, addressing the general problem of arresting transmission of vibrations and other movements from an external source to a machine has become more important.
An example of a machine technology in which demands on accuracy and precision are extreme is microlithography as used, for example, in the manufacture of microelectronic devices (e.g., integrated circuits). Microlithography involves the transfer of a pattern, used to define a layer of a microelectronic device, onto a sensitized surface of a suitable substrate such as a semiconductor wafer. Hence, microlithography is analogous to an extremely sophisticated photographic printing process. Modern microlithography systems (commonly called “steppers”) are capable of imprinting patterns in which the pattern elements, as imaged on the substrate, have linewidths at or about the wavelength of light used to form the image. For example, certain modern steppers can form images of linear pattern elements having a linewidth of 0.25 or 0.18 μm, or even smaller, on the substrate. Achieving such a high level of performance requires that all imaging, positioning, and measuring systems of the stepper operate at their absolute limits of performance. This also requires that vibrations and other unwanted physical displacements be eliminated from the machine.
A conventional approach to vibration attenuation between two physical bodies involves the use of one or more air springs between the bodies. An air spring is a spring device in which the energy-storage element is air that is confined in a container that includes an elastomeric bellows or diaphragm. Air springs are commercially available in many different configurations and sizes and are used in a wide variety of applications with good success. A key attribute of an air spring is its reduced stiffness with respect to the load applied to the air spring. (Usually the load is applied axially relative to the air spring.) For many applications (e.g., trucks and other heavy machinery), especially in situations in which attenuation of axial motion is the objective, an air spring is sufficient for achieving satisfactory vibration attenuation.
A disadvantage of an air spring for certain applications is its relatively high lateral stiffness. The high lateral stiffness can result in significant transmission via the air spring of non-axial motions from one body to another. If the subject machine is one (e.g., a stepper) in which and/or from which substantially all vibrations must be isolated completely, an air spring will exhibit unsatisfactory performance. For example, in a stepper machine any significant lateral stiffness in a vibration-attenuation device can cause problems with overlay accuracy of different layers as imaged on a wafer. Another possible problem is an increased mean standard displacement (“MSD”) between the reticle stage and the wafer stage.
Increasing the axial length of certain types of air springs can reduce their lateral stiffness. However, making an air spring longer may render certain uses of it impossible. This problem has arisen in modem stepper machines in which, despite the large size of a stepper machine, spaces between components and assemblies of the machine are usually very tight. For example, in most stepper machines the height of the focal plane of the projection lens above the floor of the room containing the machine is dictated by the height of adjacent robotics for transporting wafers to and from the machine. The dictated height usually is about 600 mm above the floor (which is a standard elevation in the industry). This 600-mm space must accommodate the massive wafer stage and its movement mechanisms, as well as various large support members for the stage, projection lens, and other portions of the machine. Under such conditions, the remaining available space simply is inadequate for accommodating air springs sized for achieving satisfactory performance.
Hence, in modem stepper machines and related types of equipment, there is a need for vibration attenuators and analogous supporting devices that exhibit good vibration attenuation in the axial direction and that exhibit substantially zero lateral stiffness to reduce transmission of vibrations between any of various portions of the machine, especially at certain vibration frequencies.
Various examples exist in the known art for addressing the problem of achieving improved vibration attenuation in a stepper machine. For example, U.S. Pat. Nos. 6,144,442 and 6,226,075 discuss respective “supporting devices” having low stiffness in a directions parallel to a support direction and perpendicular to the support direction. Other approaches to solving this problem are discussed in U.S. Pat. No. 5,701,041 and in European Patent Publication Nos. EP 0,973,067 A2 and EP 1,160,628 A2. However, in view of the extremely demanding application to which vibration-attenuation devices are put in modem stepper machines, all of the currently known vibration-attenuation devices fall short of satisfying all performance criteria for such applications. Further improvement is needed.